In this type of technical field, it is proposed to perform handover among a plurality of radio access networks or mobile communication systems based on same or different types of radio access technologies (RAT). For example, it can be considered that, after a user apparatus completes a telephone call in a radio access network, the user apparatus enters waiting operation in another radio access network of a different frequency. By doing that, when the radio access network is becoming congested, load balancing of the network can be achieved, so that the accommodation number and the throughput in the whole of the plurality of systems can be improved.
FIG. 1 shows a situation in which handover is performed among the same or different RATs. In the example shown in the figure, a third generation system (3G-RAT) including a radio access network of a first frequency f1 and a radio access network of a second frequency f2 coexists with a second generation system (2G-RAT) that uses a third frequency f3 which is different from them in a similar geographical area. The 2G-RAT may include a system such as GSM and PDC, for example. In the example shown in the figure, a user apparatus UE that completes a telephone call using the first frequency f1 enters waiting operation in the second frequency f2 or in the 2G-RAT. A handover may be performed between a system of the long term evolution (LTE) scheme and a system of the UMTS scheme.
For example, the non-patent documents 1 and 2 disclose handover among the same or different types of RATs.    [Non-patent document 1] 3GPP, TS25.331    [Non-patent document 2] 3GPP, TS25.304